Decomposable twofold triple systems with non-Hamiltonian 2-block intersection graphs

TL;DR

This paper constructs large twofold triple systems with bipartite, connected, non-Hamiltonian 2-block intersection graphs, providing counterexamples to a long-standing conjecture and expanding understanding of their structural properties.

Contribution

It demonstrates the existence of such systems for all sufficiently large v congruent to 1 or 3 mod 6, using novel embedding and trade techniques.

Findings

Existence of TTS(v) with bipartite, connected, non-Hamiltonian 2-BIG for all large v

Construction method involves embedding smaller systems within larger ones

Provides counterexamples to Tutte's 1971 conjecture

Abstract

The 2-block intersection graph (2-BIG) of a twofold triple system (TTS) is the graph whose vertex set is composed of the blocks of the TTS and two vertices are joined by an edge if the corresponding blocks intersect in exactly two elements. The 2-BIGs are themselves interesting graphs: each component is cubic and 3-connected, and a 2-BIG is bipartite exactly when the TTS is decomposable to two Steiner triple systems. Any connected bipartite 2-BIG with no Hamilton cycle is a counter-example to a conjecture posed by Tutte in 1971. Our main result is that there exists an integer $N$ such that for all $v\geq N$, if $v\equiv 1$ or $3\mod{6}$ then there exists a TTS($v$) whose 2-BIG is bipartite and connected but not Hamiltonian. Furthermore, $13<N\leq 663$. Our approach is to construct a TTS($u$) whose 2-BIG is connected bipartite and non-Hamiltonian and embed it within a TTS($v$) where $v>2u$ in such a way that, after a single trade, the 2-BIG of the resulting TTS($v$) is bipartite connected and non-Hamiltonian.